ABSTRACT Cell migration can be divided into three mechanical processes: i) protrusion; ii) adhesion and deadhesion; and iii) cytoskeleton contraction. These processes are coordinated by spatially and temporally organized regulatory signals, many of which are mechano-responsive, i.e. the signaling events are modulated by the mechanical forces they regulate. As the first of the three processes protrusion defines the directionality and persistence of cell movements. The protrusion machinery is also the integrator of internal and external guidance cues, including polarity, chemotactic, haptotactic, and durotactic stimuli. Accordingly, cell protrusion in itself is the result of complex intersections between mechanical and chemical signaling pathways that translate the various inputs into the coordinated activation of pathways that push the cell edge forward. While it is safe to assume that the molecular constituents of these pathways are largely known, we have very little understanding of the mechanism of pathway integration. Here we focus on actin-based protrusion. Dissecting the system of pathways that regulate actin-based protrusions remains a formidable challenge because of the substantial functional overlap and non-linear (feedback and feed forward) relations between pathways. Conventional perturbation approaches, which disrupt one pathway at a time, yield largely uninterpretable results because the overall pathway system immediately adapts. The overarching theme of this proposal is therefore the development of an analytical framework that relies on measuring basal fluctuations of pathway activities in molecularly unperturbed and spontaneous cell protrusions and on statistical inference of the functional hierarchy and timing among intersecting pathways using stochastic time series analysis. The proposed analytical framework will provide unprecedented data on the topology and kinetics of information flows in non-linear and partially redundant pathway systems. While cell protrusion is a particularly attractive problem to develop and demonstrate the power of basal fluctuation analysis the analytical tools to be developed will be general and thus applicable to other cell biological investigations. Building on our findings of the previous funding period we will apply the analytical framework to test hypotheses on the coordination of pathways that initiate and upregulate actin filament assembly in persistent protrusion events during directed migration.